A four-stroke combustion engine is disclosed in German patent publication 30 22 901. The intake channel opens exclusively into the crankcase whereby the mixture drawn into the crankcase during the downward movement of the piston is conveyed via a transfer channel into a valve housing from where it flows through the intake valve into the combustion chamber. This mixture passage ensures that the entire air-fuel-lubricant mixture, before entering the combustion chamber, flows through the crankcase and the connecting channels so that the moving parts are lubricated. However, this guiding of the mixture has the disadvantage of causing undesirable heating of the mixture resulting in efficiency losses because of the volumes to be conveyed.
German patent publication 34 38 031 describes a four-stroke engine in which the intake channel of the intake valve is connected directly to the mixture-preparation device. A mixture conduit branches off the intake channel to the crankcase which is connected to the valve housing via a further connecting line parallel to the cylinder wall. The valve housing, in turn, is connected by a connecting channel to the intake channel so that a closed, mixture-conveying circulation is provided, which includes the crankcase and the valve housing. For controlling the mixture-conveying circuit, the mixture conduit to the crankcase is provided with an intake valve, and the connecting channel between the valve housing and the intake channel has a throttle flap. The resulting control of the mixture-conveying lubricant circuit is complex.